sndstat: nvlist schema and API definition changes

• SNDSTAT_LABEL_* are renamed to SNDST_DSPS_*, and SNDSTAT_LABEL_DSPS becomes SNDST_DSPS.
• Centralize channel number/rate/formats into a single nvlist The above nvlist is named "info_play" and "info_rec"
• Expose only encoding format in pfmts/rfmts. Userland has no direct access to AFMT_ENCODING/CHANNEL/EXTCHANNEL macros, thus it serves no meaning to expose too much information through this pair of labels. However pminrate/rminrate, pmaxrate/rmaxrate, pfmts/rfmts are deprecated and will be removed in future.

This commit keeps ioctls ABI compatibility with __FreeBSD_version
1400006 for now. In future the compat ABI with 1400006 will be removed
once audio/virtual_oss is rebuilt.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D29770